1. Field of the Invention
The present invention relates to supports for lithographic printing and to a process for producing the same. Particularly, it relates to aluminium plates having a rough surface for lithographic printing plates and a process for producing the same.
2. Description of the Prior Art
In order to use aluminum plates as supports for lithographic printing plates, the surface is generally roughened to improve adhesion to a sensitive layer provided thereon and to improve the water retention property. The treatment for roughening the surface is called graining, and mechanical graining such as ball graining, wire graining, brush graining, etc., and electrochemical graining, i.e., electrolytic etching have been known.
Since the aluminum surface grained by these processes is comparatively soft and easily abraded, it is subjected to anodic oxidation to form an oxide film thereon. The resulting surface of the processed aluminim plate is hard and exhibits excellent abrasion resistance, good affinity for water, good water retention and good adhesion to the photosensitive layer. However, when anodic oxidation alone is carried out after the graining directly after the mechanical graining, the oxide film is blackened by insoluble substances (for example, the remnants of abrasives, aluminium cutting dusts or brush fragments, etc.) formed during graining treatment and, consequently, the appearance is damaged. Further, the sensitivity of a sensitive layer applied thereto deteriorates or becomes uneven, the visual contrast between the image and the non-image areas is poor even with development and, consequently, and further operations are indispensable for plate making, such as correction or elimination, etc. Moreover, not only operations in printing, for example, control of water/ink balance, etc., become difficult, but also the non-image area is easily contaminated. For this reason an intermediate treatment is generally carried out after graining and before anodic oxidation. As the intermediate treatment, a chemical etching process is described in U.S. Pat. No. 3,834,998 and an electrochemical etching process is described in U.S. Pat. No. 3,929,591. In the case of electrochemical etching, though the treated surface is somewhat white compared with the aluminium surface subjected to graining and then anodic oxidation, halation does not substantially occur and the sensitivity is low and interferes with post-development operations such as correction or elimination of images. Further, printing operations (e.g., adjustment of balance between water and ink, etc.) become difficult to accomplish.
In the case of chemical etching, though contamination of the non-image area and resistance to scratching are improved, a fine grain structure formed on the aluminium surface is broken. Therefore, the excellent surface characteristics of the graining treatment deteriorate, water retention and printing durability become inferior and fine halftone dots of 1 to 5% cannot be reproduced.